Its an expression or term used when someone is in an extreme disgust or distaste.
Its an interjection of surprise reaction because of something gross.
Its a cuter way or saying "yuck" "ouch" and "yikes"
A: omg, wth I don't believe in god ugh.
B: oh noes , why call him if yah don't believe in him... yanno you're sooo youchie!